1. Field of the Invention
The present invention relates to threaded tubular joints or connections and to a thread form for threaded connections of the type used for securing tubular flow conduits to form a desired continuous flow path.
2. Description of the Prior Art
A variety of threaded connections are known in the prior art for joining tubular flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid, gases or a combination of both. Typical examples of such flow conduits include casing, expandable casing, tubing, drill pipe and risers for oil, gas, water and waste disposal wells, and in horizontal and trenchless drilling applications. In the case of oil field casing and tubing, it is a common practice to use pipes of a definite length, with sections of pipe joined to form a string. The string of pipes effectively creates one lengthier pipe, intended to provide a means to reach the depth at which the reservoirs of gas or oil are found in order for extraction to the surface.
The pipe sections are secured together at their ends by an externally threaded connector, or “pin” that is threadedly received within an internally threaded connector or “box”. Typically, each pipe section has a pin on one pipe end and a box at the opposite pipe end. Some pipe has an internally threaded coupling secured to one end of a double pin pipe section to produce the box. The individual pipe sections are frequently referred to as a “pipe joint”. Tubing and casing pipe joints are usually 30 ft. in length but can vary in length from 2 ft. to 40 ft. or longer.
The various pipe strings used in constructing a well are usually assembled on the floor of a drilling or workover rig. The pipe string is lengthened and lowered into the well as succeeding pipe joints are added to the string. During this assembly procedure, the pipe joint being added to the string is lowered, pin down or pin up, into an upwardly or downwardly facing box projecting from the drilling rig floor. This procedure is commonly referred to as “stabbing” the pin into the box. After being stabbed, the added pipe joint is rotated to engage the threads of the pin and box, securing the joint to the string.
The connections for strings of drill pipe, tubing or casing must be able to withstand the total weight of a string of pipe many thousands of feet long. Since the drill string must also be used for the purpose of drilling, the joints must be able to withstand high torque loads, as well. Additionally, wells may not be driven in exactly vertical fashion or even in straight line fashion. Horizontal drilling operations are common today. The tubing used to drill the well and/or convey fluid from the well must be able to follow the course of the well as greater lengths are reached. This places bending loads on the drill, casing or tubing strings at various places along the length thereof. Sections of tubing or casing are sometimes driven into the well, resulting in axial compressive loads being exerted. Thus, in addition to withstanding tremendous tensile loads, the threaded connections in the pipe string must also be able to absorb considerable compression loads.
There have been numerous advances in thread technology of the type under consideration in recent years. For example, Re. U.S. Pat. No. 30,647 issued to Blose in 1981 disclosed a tubular connection having a thread form which provided an unusually strong connection while controlling the stress and strain in the connected pin and box members of the connection. The thread form featured mating helical threads which were tapered in thread width in opposite directions to provide wedge-like engagement of the opposing flanks to limit rotational make-up of the connection. The wedge thread, if properly designed, provides high torsional resistance without inducing axial or radial stresses into the tubular connection upon make-up of the joint, making it easier to break out the joints if this becomes necessary. By reducing axial or radial stresses in the threaded connection, a sounder connection is theoretically provided which is able to withstand a greater level of operating stress and strain.
U.S. Pat. No. 4,600,224, issued Jul. 15, 1986 to Blose was a refinement and further improvement to the basic wedge thread concept. In the invention disclosed in the '224 patent, a connection was shown having a “chevron” load flank. Radial make-up of the threaded connection was controlled by the special thread structuring where the radial movement of a thread into a mating thread groove was restricted by a chevron type interfit between two load bearing thread surfaces of the threaded connection instead of relying upon thread width alone.
Re. U.S. Pat. No. 34,467 issued Dec. 7, 1993 to Reeves purported to be an improvement to the basic Blose wedge thread design. As explained by the patentee, when Blose's connection is rotatably made up to engage both the front and back thread flanks, incompressible thread lubricant or other liquid may be trapped between the engaged load flanks. This trapped thread lubricant can resist the make-up torque and give a false torque indication that results in lower than desired stress and strain being induced in the Blose connection and reducing the design strength and load carrying capacity. The invention described in Re. U.S. Pat. No. 34,467 purports to preclude the possibility of false indication of torque by excluding thread lubricant from between the thread load flanks that are brought into engagement at make-up.
In Re. U.S. Pat. No. 30,647 and Re. U.S. Pat. No. 34,467, the preferred threads were “dovetailed-shaped” in cross section, being wider at the crests than at the roots. U.S. Pat. No. 4,600,224 was a departure from the Blose design in that a semi-dovetail or partial dovetail thread was disclosed. However, the thread crest width continued to be greater than the thread root width as in the traditional definition of the term “dovetail.”
U.S. Pat. Nos. 6,254,146 and 6,722,706, to Kris L. Church, were directed to further improvements in thread forms of the type under consideration. The thread forms shown in these earlier Church patents include a special thread structuring where the radial movement of one thread into a mating thread groove is controlled by a complex profile interfit between the two mating thread surfaces of the threaded connection. The complex profile can be present on the stab flank, on the load flank, or on a combination of the two flanks. A controlled clearance is provided between the mating crests of the interengaged threads to prevent hydraulic pressure buildup caused by entrapped lubricant between the thread crests and roots. The stab and load flanks complex profiles are preferably multi-faceted flanks, each having at least three facets and four radii per flank. The pin thread crests have a crest width and the pin roots have a root width. The width of the crest is less than the width of the roots, which is exactly opposite that of the general dovetail design.
Despite the improvements in thread form design discussed above, a need continues to exist for an improved thread form which is capable of coupling tubular pipe sections quickly and efficiently, and which forms a secure connection.
A need also exists for an improved thread form for coupling of tubular pipe sections that maximizes the connection's “real-estate area” available in the geometry of the design to thereby maximize the desired properties of the connection for a given connection length.
A need also exists for such a thread form which minimizes the amount of thread material cut away during the threading operation performed in machining of the thread form.
A need exists for such a connection which is high in tensile strength, which can withstand extremely high torque, and which features fast make-up characteristics.
A need also exists to design a short length connection, using such an improved thread form, without sacrificing the integrity of the connection.
A need also exists to design a connection on plain end pipe, using such an improved thread form, that has comparable performance properties of a more expensive forged upset designed connection.